1. Field of the Invention
The present invention relates to a method for producing a low chlorine-containing and highly pure glycidyl methacrylate which substantially contains no epichlorohydrin.
2. Discussion of the Related Arts
Glycidyl methacrylate is usually produced by allowing methacrylic acid to react with epichlorohydrin to yield the chlorohydrin ester of methacrylic acid and subjecting the ester to dehydrochlorination. Alternatively, it is produced by the reaction of an alkali metal salt of methacrylic acid with epichlorohydrin in the presence of a quaternary ammonium salt as a catalyst.
The glycidyl methacrylate produced by these methods, however, contains residual chlorine compounds such as epichlorohydrin at 0.1 to 1.0% by weight, resulting in a decreased purity of glycidyl methacrylate and an increased content of free chlorine.
In order to decrease the content of free chlorine from epichlorohydrin and other chlorine compounds, several methods have been disclosed, which include the method in which azeotropic distillation is performed in two stages (French Patent No. 2656305) and the method in which distillation is performed after stripping with an oxygen-containing gas in the presence of a quaternary ammonium salt (Japanese Patent Laid-Open No. 4-187682). These methods, however, does not sufficiently accomplish their object because not less than 100 ppm of epichlorohydrin, etc. remains in the final product. The increase in the number of process steps due to these additional treatment is also a problem. On the other hand, it is known that the content of chlorine can be reduced by performing a treatment with a hetero poly acid after completion of the reaction and then performing distillation (Japanese Patent Laid-Open No. 63-255273). This method, however, uses an expensive hetero poly acid, and therefore a more economical method is desired.
Also, there are some theoretically chlorine-free methods based on an ester exchange reaction of methacrylic acid ester and glycidol (Japanese Patent Laid-Open Nos. 52-25714, 54-3007, 56-118075, 55-102575 and 55-94379 and many other publications). These methods, however, are not suitable for industrial production of glycidyl methacrylate because glycidol, the starting material, is so unstable in storage that its content decreases over time even at room temperature.
Other disclosed methods for production of glycidyl methacrylate include the method in which an allyl methacrylate is epoxidated with hydrogen peroxide [European Patent No. 190609, Chim. Ind. 72, 610-616 (1990)] and the method in which epoxidation is performed using a microorganism (Japanese Patent Laid-Open No. 61-260893). These methods, however, are not suitable for industrial production because the starting material does not suit for industrial use and because these methods are not economically favorable.
In conventional methods for production of glycidyl methacrylate, the reaction of an alkali metal salt of methacrylic acid with a 3-10 times higher molar amount of epichlorohydrin are carried out in the presence of 0.3 to 2.0 mol % of a quaternary ammonium salt as a catalyst and about 0.01 to 0.2% by weight of a polymerization inhibitor at a reaction temperature of 80.degree. to 120.degree. C. for 1 to 5 hours, and the by-produced salts are removed by filtration, washing with water and other means to yield crude glycidyl methacrylate, which is finally purified by distillation under reduced pressure.
The purified glycidyl methacrylate thus obtained contains 0.1 to 1.0% by weight of residual epichlorohydrin and at most 98.5% by weight of glycidyl methacrylate, showing a decreased purity of glycidyl methacrylate and an increased content of free chlorine.
In particular, acceleration of corrosion of metal substrate of electronic equipment by the increased free chlorine is a problem, because the main applications of glycidyl methacrylate include electronic materials such as paints and resists. Moreover, epichlorohydrin has toxic effects such as intense skin irritation, requiring special caution in handling glycidyl methacrylate which contains residual epichlorohydrin. In addition to these problems in safety, hygiene and environmental protection, there are also legal regulations.